1 ASTP MMRMOTIVATIONDevelopment of next generation space exploration vehicles and space structures require high temperature materials withLow densityHigh strength and ductilityOxidation resistanceGood creep propertiesMetal Matrix Composites based on intermetallics such as gamma-titanium aluminides (-TiAl) have been identified as material of choice for aerospace applications in the temperature range of 600oC to 900oC.-TiAl have been identified as possible replacement for superalloys in engine components and nozzles due to their high specific strength and oxidation resistance at high temperatures.

2 STATE OF THE ART IN TITANIUM ALUMINIDESASTP MMRSTATE OF THE ART IN TITANIUM ALUMINIDESCurrent state of the art manufacturing techniques have produced -TiAl based alloys withStrength of 1 GPaDensity of 3.8 gm/cm3Thus, they have posed a stiff competition for superalloys which haveStrength of ~1.2 GPaDensity of ~8 gm/cm3In order to capitalize on these advancements on -TiAl, further work is needed in the areas ofNear-net shape manufacturingLow cost material productionMaterials property databaseTi-45Al-X(Nb, B, C)Draper et al, 2003Example of advanced concept for TiAlBartolotta, et al 1999

3 Hexagonal Closed PackedASTP MMRINTERMETALLICSHigh strength compounds of metals whose crystal structures are different from the constituent metals.They form because the strength of bonding between unlike atoms is greater than that between like atoms.Examples are TiAl, Ti3Al, TiAl3, Ni3Al, Co3Ti.AlTiTiAlFace Centered CubicStructureTi3AlHexagonal Closed PackedStructure

4 PHASES OF TITANIUM ALUMINIDESASTP MMRPHASES OF TITANIUM ALUMINIDES-TiAl can exist in two different phasesPure -TiAl phaseMixture of -TiAl and 2-TiAlPure -TiAl has high strength and oxidation resistance, but it shows almost no ductility. Thus, not much research has been done on pure -TiAl.Mixture of -TiAl and 2-TiAl has high strength and good ductility, but does not show good oxidation resistance.But the properties of this phase can be improved byControl of its microstructureSmall additions of TiB2, Nb, and Cr.A lot of research has been concentrated on this phase of TiAl.Phase diagram -Titanium Aluminide

5 -TiAl MICROSTRUCTURESASTP MMR-TiAl MICROSTRUCTURESLamellar MicrostructureDuplex MicrostructureTwo main characteristic microstructures possible in -TiAl.Duplex Microstructure: Exhibits good strength and ductility.Lamellar Microstructure: Has good creep properties.These microstructures can be produced with appropriate heat-treatments.Refinement and control of grain size of these microstructures have shown improved mechanical properties.

6 MANUFACTURING TECHNIQUESASTP MMRMANUFACTURING TECHNIQUESThe state of the art manufacturing techniques of -TiAl involve ingot metallurgy and extrusion processes, which are often time consuming and expensive.Other methods follow the powder metallurgy route such asSinteringHot PressingHot Isostatic pressingPowder consolidation methods usually have the advantage of yielding near-net shape parts.But the methods mentioned above require exposure to high temperatures for long time to achieve full densification.Such extended exposure at high temperatures leads to grain growth and deterioration in mechanical properties. Controlling or minimizing grain growth has long been known to increase strength and ductility of materials.Rapid consolidation can be a potential solution since it generally reduces segregation, refines microstructure and thus produces a more homogeneous material.

13 CHEMICAL COMPOSITION Phase DiagramASTP MMRCHEMICAL COMPOSITIONChemical composition analyses of the CERAC/ESPI powders and consolidated samples revealed the chemical composition as Ti-49.5(at%)Al.Presence of alpha2 phase is very less in this composition.In order increase alpha2 composition, the aluminum must be decreased up to 46% to 48%New powders were procured from Oakridge National Laboratories with 46% Al and additions of Nb and Cr.Phase Diagram

14 Density (gm/cm3) TiAl Sample DENSITYASTP MMRDENSITYDensity (gm/cm3)TiAl SampleThe average density of the consolidated samples was found to be ~ 3.9 gm/cm3.The density of the gamma phase is 3.76 gm/cm3, while that of the alpha2 phase is 4.2 gm/cm3.The theoretical density of the samples will be determined by calculating the amount of alpha2 phase present in the sample.From the micrographs and the density data, the consolidated samples seem fully dense.

20 COMPARISON WITH STATE OF THE ARTASTP MMRCOMPARISON WITH STATE OF THE ARTStrength (MPa)P2C consolidatedDraper, et al 2003Temperature (Celsius)TemperatureP2C consolidated(flexure Strength)As-extruded(tensile strength)400oC1600 MPa1100 MPa800oC1000 MPa700 MPa950oC800 MPa500 MPaHigh temperature mechanical properties of P2C consolidated TiAl were comparable to that of TiAl produced by extrusion process by Draper et al, 2003.